An automotive automatic transmission generally includes a transmission mechanism having planetary gear units which include shift change elements (hereinafter referred to as gears) such as sun gears and planetary carriers and having hydraulic friction engaging elements such as hydraulic wet-type multiple disk clutches and hydraulic band brakes (hereinafter referred to as clutches and brakes, respectively). In an automatic transmission of this type, the gear connection is changed over by releasing one(s) of the friction engaging elements associated with shift change and by engaging the other friction engaging element(s) associated with shift change, to select those gears which contribute to torque transmission, to thereby establish a desired shift position.
In recent years, in order to improve the drivability of automobiles and reduce the fuel consumption, attempts have been made to further enhance the degree of electronic control for automatic transmissions and increase the number of shift positions or transmission stages. A typical multi-stage automatic transmission includes a main transmission mechanism constructed by an existing transmission mechanism and a subsidiary transmission mechanism coupled in line, in respect of torque transmission, with the main transmission mechanism. Those gears of the two transmission mechanisms which contribute to torque transmission are combined in various manners, to thereby establish an arbitrary one of a required number of shift positions, e.g., five forward shift positions and one reverse shift position. For example, the gear connection in the main transmission mechanism is changed over to effect a shift change operation among the first, second and third shift positions, and the gear connection in the subsidiary transmission mechanism is changed over to effect a shift change operation between the third and fourth shift positions. Further, to carry out a shift change between the fourth and fifth shift positions, the gear connection in the main transmission mechanism is changed over, with the gear engagement state (torque transmission path) in the subsidiary transmission mechanism set so as to correspond to the fourth shift position. In other words, by use of the subsidiary transmission mechanism, a switching is made between the lower-speed shift positions including the first through third shift positions and the higher-speed shift positions including the fourth and fifth shift positions.
In an electronic controlled automatic transmission, a shift map determined as a function of vehicle speed and throttle valve opening degree, as shown in FIG. 13, is generally used to select a shift position. From this map, an optimum shift position (target shift position) suitable to detected values of the vehicle speed and throttle valve opening degree is selected. In the case of a kick-down at a rapid acceleration, the target shift position is generally determined by the throttle valve opening degrees. That is, when the throttle valve opening degree traverses the 5-4 shift line or 4-3 shift line shown in FIG. 13, a down-shift command is output. As a result, if the accelerator pedal is depressed by the driver and the throttle valve opening degree .theta. reaches the point B from the point A in FIG. 13, a down-shift is carried out from the fifth shift position to the fourth shift position. When the throttle valve opening degree .theta. reaches the point C from the point A, a so-called skip down-shift from the fifth shift position to the third shift position is effected.
On an occasion that a direct down-shift from the fifth shift position to the third shift position is effected in the aforementioned multi-stage automatic transmission, that is, a skip down-shift is effected, it is necessary to make the changeover of gear connection in both of the main and subsidiary transmission mechanisms. However, to simultaneously control a plurality of transmission mechanisms, an advanced technique such as the modern control theory must generally be used.
Further, in the automatic transmission, the shift control is generally effected based on the outputs of two rotation speed sensors respectively indicating the rotation speeds of the input and output shafts of the transmission. However, the shift change condition in each of the main and subsidiary transmission mechanisms cannot be detected based on the outputs of the two sensors. Therefore, it is extremely difficult to simultaneously control the shift change operations in both of the transmission mechanisms based on the two sensor outputs. Provision of additional rotation speed sensors makes it possible to detect the shift change condition of each transmission mechanism, and, in turn, to simultaneously control a plurality of transmission mechanisms. In this case, however, the cost rises.
Conventionally, therefore, in order to carry out a skip down-shift, the gear connection in the main transmission mechanism is first changed over, and then the changeover of gear connection is made in the subsidiary transmission mechanism. For example, for the down-shift from the fifth shift position to the third shift position, a method has been adopted in which the fourth shift position is temporarily established during the downshift, so that the downshift is carried out by way of the fourth shift position. However, this type of shift control method entails the following defects.
As is well known in the art, the release of a clutch or brake, which is a hydraulic friction engaging element, requires a predetermined period of time corresponding to a response delay in oil pressure release. Therefore, even if a down-shift command for instructing a shifting to the third shift position is issued immediately after the temporary establishment of the fourth shift position, the fourth shift position is kept established until the predetermined time period elapses because of the presence of the delay in oil pressure release. Further, to establish the fourth shift position, it is necessary to make a time-consuming determination as to the synchronization of rotation speeds of the input and output shafts of the transmission. According to the conventional method in which a skip down-shift is made by effecting a one-step down-shift plural times, a shift shock occurs each time the one-step down-shift is effected and a period of time required for shifting becomes longer. Therefore, the ride qualities and shift response are degraded.